Determining Seasonal Variability in the Source and Age of Carbon transported by the Mekong River

Project Strategy Overview

      The objective of the current iresearch s to examine regional-scale landscape dynamics in river basins in Southeast Asia, relative to their connectivity to the South China Sea, with an emphasis on the Mekong River.  The geographic and geopolitical provenance of this project covers a diverse set of environments, being subject to rapid changes. Over the past year, it has become clear  that the Mekong River may be subject to rapid development of hydropower, where not only the upper "Chinese Cascade" of dams is in place, but that an additional 12 mainstem dams are under discussion, along with a projected 97 (!) on tributaries in Laos.  The cumulative effect of these dams would fundamentally alter the overall hydrologic and biogeochemical regime of the Mekong, (as well as fisheries and livelihoods) with consequences for the South China Sea. This project is in the position of both fundamental science challenges to understand what this might mean, as well as in a position of responsibility for not only analyzing the potential outcomes, but for making the results more broadly known.

    Our focus is on "high resolution but regional" work A basic premise is that the understanding of regional scale processes requires the higher resolution now possible with satellite data, process-based models, and field measurements, with the convergence of data and models from multiples sources. The work can be significantly extended, with the inclusion of information from specialized local and regional institutions. But such information is typically not readily accessible. A key regional player is the Mekong River Commission. Through a formal Letter of Agreement in place with the MRC, we are jointly developing a "Virtual Mekong Basin," to provide a portal to the world what the current and projected status is of the basin, under different conditions. This agreement opens the door further, to important data resources, as well as providing a portal for the use of NASA-information to broader communities.

      Our strategy has been to establish the overall climatology and hydrology of the broader region, then dialing down through to the biogeochemistry and ecosystem productivity of the Mekong and the Tonle Sap Lake, culminating in the hydrologic and chemical export to the sea. All of this has taken considerable time to assemble, with the not-unexpected hiccups along the way (especially for such a challenging geopolitical region). In the following sections we summarize thework on biogeochemistry.

  Biogeochemistry

      Rivers transport dissolved inorganic carbon (DIC) and organic carbon (OC) from land to the ocean, but they also chemically transform this carbon during its transit through in situ respiration and photosynthesis, and exchange CO₂ with the atmosphere through gas exchange. Most rivers are supersaturated with CO₂, and as such are a net source of carbon to the atmosphere on the order of 1 Pg C/year.  However, this value is poorly constrained and thought to be an underestimate. This CO₂ supersaturation in rivers is driven by two processes, advection of CO₂ produced by respiration in soil of their watersheds and/or hyporheic zones and the prevalence of net heterotrophic metabolism in rivers. Tropical rivers carry two-thirds of the global riverine organic carbon load, and many of them experience strong seasonal flooding, bringing large amounts of allochthonous carbon into the river. This high organic carbon load combined with high respiration rates in tropical climates creates the potential for high CO₂ outgassing rates in tropical rivers. 

      Few studies have investigated seasonal and interannual variations in the pCO₂ of large tropical rivers. We investigated the cumulative sequences of river metabolism organic carbon composition, and outgassing in the Mekong, as a large, tropical, carbonate-rich river influenced by a significant flood-drought cycle. Alin et al (2011) examined the physical controls on outgassing, with the all-important evaluation of the gas transfer velocity. An important parameter linking sediment transport to organic carbon is the changes in the composition of particulate organic matter (POM) over the course of the hydrograph. Ellis et al. (in review) examined the changes in the elemental and lignin compositions of POM, showing pronounced seasonal differences. Ellis et al. (in prep, near final) carried the compositional analysis further, to establish a baseline for terrestrial versus marine derived OM in coastal sediments, using a new proxy, the branched/isoprenoid tetraether (BIT) index. Finally, Ellis et al (draft) assessed the time scales over which terrestrial organic matter is exported from the Mekong watershed via fluvial processes, by looking at the ∆¹⁴C of the lignin phenols. Lockwood et al. (about to be submitted) established annual carbon budgets for the lower amazon, and the sequence of metabolic processes producing those budgets.

Alin, S. R., M. F. F. L. Rasera, C. I. Salimon, J. E. Richey, G. W. Holtgrieve, A. V. Krusche, and A. Snidvongs (2011). Physical controls on carbon dioxide transfer velocity and flux in low‐gradient river systems and implications for regional carbon budgets, J. Geophys. Res., 116, G01009, doi:10.1029/2010JG001398

      Outgassing of carbon dioxide (CO₂) from rivers and streams to the atmosphere is a major loss term in the coupled terrestrial‐aquatic carbon cycle of major low‐gradient river systems (the term "river system" encompasses the rivers and streams of all sizes that compose the drainage network in a river basin). However, the magnitude and controls on this important carbon flux are not well quantified. We measured carbon dioxide flux rates (FCO₂), gas transfer velocity (k), and partial pressures (pCO₂) in rivers and streams of the Amazon and Mekong river systems in South America and Southeast Asia, respectively. FCO₂ and k values were significantly higher in small rivers and streams (channels <100 m wide) than in large rivers (channels >100 m wide). Small rivers and streams also had substantially higher variability in k values than large rivers. Observed FCO₂ and k values suggest that previous estimates of basinwide CO₂ evasion from tropical rivers and wetlands have been conservative and are likely to be revised upward substantially in the future. Data from the present study combined with data compiled from the literature collectively suggest that the physical control of gas exchange velocities and fluxes in low gradient river systems makes a transition from the dominance of wind control at the largest spatial scales (in estuaries and river mainstems) toward increasing importance of water current velocity and depth at progressively smaller channel dimensions upstream. These results highlight the importance of incorporating scale‐appropriate k values into basinwide models of whole ecosystem carbon balance.


Ellis, E.E., R.G. Keil, A.I. Ingalls, and J.E.Richey. (in review). Seasonal variability in the sources of  particulate organic matter of the Mekong River as discerned by elemental and lignin analyses.  J. Geophys. Res. Biogeosciences (2011JG001816).

      The Mekong River ranks within the top ten rivers of the world in terms of water discharge and sediment load to the ocean, yet its organic matter (OM) composition remains unstudied. This river is experiencing anthropogenically-forced changes due to land use and impoundment, and these changes are expected to intensify in the future. Accordingly, we monitored bulk OM composition and vascular-plant signatures (using lignin phenols) of Mekong River particulate OM (POM) over a one-year period. Autochthonous production comprises a greater proportion of POM during the dry season than in the rainy season, as demonstrated by higher percent organic carbon values (7.9 ± 2.4 vs. 2.2 ± 0.4%), lower yields of lignin normalized to carbon (0.40 ± 0.05 vs. 1.1 ± 0.3 mg (100 mg OC)-1, and an increase in N:C ratios towards phytoplankton values during the dry season (from 0.06 to 0.11). Changes in the lignin-phenol composition of POM suggest that gymnosperms contribute more toward OM composition during the dry season, with angiosperms dominating in the wet season. This is supported by calculations of the lignin phenol vegetation index of riverine OM, which is statistically different among seasons (dry: 29.4 ± 15.6 vs. wet: 74.6 ± 15.6). These changes likely reflect seasonal differences in the proportion of flow that is coming from the Upper and Lower Basin, corresponding to compositional differences between the vegetation of these regions. Therefore, this work provides a baseline understanding of OM variability that can be used to assess how future change will affect this river.

Ellis, E.E., A.I. Ingalls,  L.T. Truxal, R.G. Keil, and J.E.Richey (in prep. Draft in near-final revision). Sources and Temporal Variability of Branched and Isoprenoid Tetraether Lipids Exported by a Large Tropical River

      The organic carbon (OC) discharged by rivers is a key component of the global carbon cycle due to the potential for this material to be permanently buried in marine sediments. Accurate accounting of the proportion of terrestrial verses marine-derived organic matter preserved in coastal sediments is of considerable interest due to the paradox that less terrestrial carbon is preserved in ocean sediments than that which is predicted from riverine fluxes.  Recently, a new proxy has been developed to trace the proportion of terrestrially verses marine-derived organic matter in coastal sediments, known as the branched/isoprenoid tetraether (BIT) index. Although the BIT index has since been widely applied to assess the proportion autochthonous verses allochthonous carbon buried in marine sediments, the variability, and especially the seasonal variability, of the BIT index in the suspended sediment carried by rivers remains largely uninvestigated.  The objective of this study was to determine the variability of the BIT index of suspended sediments carried by the Mekong River, in Cambodia, just upstream of the delta, over the course of one year. Results showed that branched GDGTs can no longer be considered to be produced strictly in soils.  In this study, between 48 to 73% of the branched GDGTs associated with suspended and bed sediment of the Mekong River were intact, indicating that the cells from which these biomarkers are derived are viable.  Further, the percentage of intact branched GDGTs found in river bed and lake bed sediments, and suspended sediments generally exceed that found in soils.  Therefore, the significant in situ production of branched, in addition to isoprenoid, tetraether lipids suggests that aquatic environments substantially modify the original BIT index imparted on sediment in upland environments.

Ellis, E.E., J.E.Richey, R.G. Keil, A.I. Ingalls, G. dos Santos, and E. Druffel. (in prep, first draft outlined). Seasonal Variability of the ∆¹⁴C of Organic Matter Carried by the Mekong River:  Insights from Radiocarbon Analysis of Lignin Phenols.

      The source and age of dissolved and particulate organic matter that is transported by the Mekong River provides dynamical information that bulk concentrations alone do not. We are assessing the time scales over which terrestrial organic matter is exported from the Mekong watershed via fluvial processes, and determining how seasonality affects the type of organic carbon being exported.  Lignin-derived phenols are a biomarker for vascular plants, as they are diagnostic of different plant material types and diagenetic condition, they are abundant, and they are fluvially transported. With a recently-developed technique, it is possible to radiocarbon date individual lignin phenols. Measurements were made from February 2009-February 2010, and analyzed for δ¹³C (organic matter source) and Δ¹⁴C (age), in the lower Mekong, near Phnom  Penh. Fine suspended sediments (FSS) and POC track each other and the hydrograph. DOC is enriched in ∆¹⁴C relative to POC, but depleted relative to lignin. Lignin results showed that plant-derived material that is characterizable as lignin is always modern..   The  ∆¹⁴C of POC is predictable and highly variable, with values ranging between -327.7 to +25.9‰, with the lowest values during low water and the highest at high water.  Such variability is consistent with the conclusion that organic carbon inputs are controlled by mountainous sources during high water and lowland and autochthonous sources during low water.

Lockwood, D., J.E. Richey, P.D. Quay, M. Sampson, and M. Ung. (in prep, to be submitted November 2011). CO₂ outgassing flux and ecosystem metabolism over an annual cycle in the Lower Mekong River  

     The main objectives of this paper are to (1) To estimate the CO₂ degassing rate in the Lower Mekong River over an annual cycle. (2) To measure river metabolism over an annual cycle using the stable isotopic tracer δ¹⁸O, which has been used to estimate the metabolic state of river systems on integrated space and time scales. (3) To determine the processes controlling the CO₂ degassing rate and observed seasonal cycles of biogeochemical variables using a combination of dissolved inorganic carbon (DIC) and O₂ mass and isotope budgets. Measurements were made at a downstream site, by Phnom Penh, tracking the hydrographic seasons. Many of the biogeochemical indices we measured at the Phnom Penh site varied seasonally and in concert with the hydrograph.  DIC and alkalinity were both negatively correlated with discharge, however, the flux of DIC and alkalinity werepositively correlated with discharge, suggesting that weathering is enhanced by precipitation during the flood season.  pH was not significantly correlated with discharge, but notably, pH was highly correlated with the calcium carbonate (calcite) saturation index, suggesting that the geology of the basin strongly influences pH, particularly the carbonate source. However, for this site to be supersaturated throughout the entire year there must be a pervasive input of CO₂ from soil or in situ respiration that keeps the water supersaturated. The seasonal trend in pCO₂ opposes the alkalinity and DIC trends, peaking in the flood season and lowest in the dry season. O₂ was undersaturated throughout our study, and O₂ and CO₂ were also inversely correlated. Together with the pCO₂ supersaturation throughout the year, these observations suggest that respiration (or advection of soil CO₂) causes the pCO₂ supersaturation.  Using the dual isotopes of O₂ and O₂ saturation measurements, calculated R:P suggested that the Phnom Penh site was net heterotrophic during the entire year, and extremely heterotrophic during the flood season. Examining δ¹³C-DIC versus δ¹⁸O provides a view of which process controls river dissolved gas concentration at a certain time-photosynthesis, respiration or gas exchange. Calculations of the  δ¹³C of CO₂ suggests that the CO₂ outgassing from the Mekong River is very close to the average signature expected for terrestrial plant material, and is slightly heavier than the δ¹³C of FPOM.  The observation that δ¹³C of outgassing CO₂ is -26‰ during the flood season suggests minimal influence of CO₂ from the respiration of freshwater plankton; this supports our observation of a high R:P ratio and the possibility that P approaches zero at this time of year. CO₂ outgassing fluxes are lower but on the same order of magnitude as flux measurements from the Amazon River basin. We calculated an annual flux of 5.4 Tg C. Although it is small compared to the outgassing contributed by the Amazon River (estimated at 0.5 Pg C/yr), this value is a significant on a global scale and is comparable to the DIC load of the Mekong River, which is 3.9 Tg C/yr. This estimate is most likely an underestimate of CO₂ outgassing in the Lower Mekong Basin because we are not including outgassing from the Mekong delta or the Tonle Sap Great Lake in this calculation.